Microchips are produced in a large number of process steps, in which changes are deliberately made within a small section of the surface of a substrate, i.e., a silicon wafer, in order, for example, to introduce trenches for deep-trench capacitors into the substrate, or to deposit thin interconnects or electrodes on the substrate surface.
To display such small structures, a mask is produced on the substrate surface so that those areas which are intended to be processed are exposed, while other areas are protected by the mask. After processing, the mask is removed from the substrate surface, for example, by incineration.
The mask is produced by applying a thin layer of a photoresist, which has a film-forming polymer as well as a photosensitive compound. This film is subsequently exposed with a mask, for example, being introduced into the beam path, which has information about the structure to be produced and which is used for selective exposure of the photoresist film. For production purposes, the mask is projected onto the photoresist film via a high-resolution lens system.
Photoresist systems are currently subject to rapid technical developments and have major financial importance. The exposure for structuring of photoresists in this case requires complex and expensive beam optics.
Difficulties can occur when radiation at a short wavelength is used to expose the photoresist. An exposure wavelength of 248 nm, and particularly at even shorter wavelengths, the high energy of the illumination radiation breaks bonds in the polymer. For example, the photon energy of 7.9 eV at 157 nm is above typical bonding energies of resist polymers, and the photon energy in the EUV band (extreme ultraviolet) with wavelengths around 13 nm is, for example, 95 eV. Polymer systems for exposure wavelengths of 248 nm and below release gaseous decomposition products, which have silicon or other decomposition products, which are damaging to the lens systems.
These decomposition products, which have silicon, can relatively slowly be converted by the residual oxygen present in the flushing gas into silicon dioxide, which can be precipitated onto the exposure optics and can “blind” them over the course of time.
Damage and contamination of the lens systems resulting from decomposition products adversely effect the optical characteristics, and thus the quality of the mask structure that is formed. This contamination may even lead to irreversible damage to the lenses. This results in replacement costs to the damaged optical systems, and maintenance costs caused by production failure.
To be able to investigate the behavior of photoresist systems during exposure and the formation of outgassing products is necessary. The corresponding investigation results may then provide opportunities to carry out chemical adaptation to the photoresist or to institute apparatus measures for protection of the lens systems.
Since the rate of development in photoresist technology is high and is increasing further, it is necessary to obtain appropriate information about the outgassing behavior of the photoresist quickly and reliably.
The connections, which are precipitated on the front lenses during the outgassing process and change their optical characteristics, are of particular interest.
It is known to irradiate photoresist systems with an electron beam in a vacuum and to gather the outgassing products by a refrigerated trap. The frozen-out materials are then vaporized separately and can be analyzed by mass spectrometry.
However, with this method, only an incomplete picture of the compounds, which are produced during the exposure process, is obtained due to the short life of some compounds or possible subsequent rearrangement or decomposition processes. Furthermore, electron bombardment cannot be transferred to exposure with photons in an unrestricted manner. Also, a large amount of time is required for this method.
Furthermore, the substances, which are released in comparable methods, are specified based on their detectability by the appropriate proof methods (gas chromatography or mass spectrometry) and their adsorption behavior on optical lenses is not detected.
The critical factor is to record the characteristic of these compounds to become fixed reversibly or irreversibly on a lens surface, and to adversely affect the optical characteristics of such a lens surface. This feature is not detected by the conventional methods and apparatuses.
An apparatus which allows the photoresist system outgassing products which are produced during exposure and are adsorbed reversibly or irreversibly on the lens surfaces to be investigated, is desirable.
Also, to preselect resist systems which are suitable for further industrial use based on emitting relatively small amounts of hazardous outgassing products, is desirable.